This invention relates generally to a substrate conditioning device for an electrophotographic printing machine and, more particularly, concerns an improvement for eliminating water spills from the ends of moisturizing rolls in the conditioning device.
In a typical electrophotographic printing process, a toner powder image is then transferred from a photoconductive member to a print sheet. In order to fix or fuse electroscopic toner material onto the print sheet by heat and pressure, it is necessary to apply pressure and elevate the temperature of the toner to a point at which the constituents of the toner material become tacky and coalesce. This action causes the toner to flow to some extent into the fibers or pores of the support mediumprint sheet (typically paper). Thereafter, as the toner material cools, solidification of the toner material occurs, causing the toner material to be bonded firmly to the support member.
One approach to heat and pressure fixing of electroscopic toner images onto a support has been to pass the print sheet bearing the toner images between a pair of opposed roller members, at least one of which is internally heated. During operation of a fixing system of this type, the print sheet to which the toner images are electrostatically adhered is moved through the nip formed between the rolls and thereby heated under pressure. A large quantity of heat is applied to the toner and the print sheet bearing the toner image. This heat evaporates much of the moisture contained in the sheet.
After the print sheet passes through the fuser, it cools and regains moisture from the environment. The substrate (e.g., paper) tends to regain a significant amount of the moisture lost while the toner regains little if any. Furthermore, as the toner cools, it contracts thereby pulling the sheet toward it, further increasing curl. Toner has a much larger coefficient of thermal expansion and a much smaller coefficient of hydro expansion than paper. Differences in the amount of toner on the different sides (e.g., a high area coverage simplex print) will cause the sheet to curl. That is, post fusing the toner contracts while the paper expands causing the print to curl, the degree of curl depends on the relative thickness of the toner layer.
A number of solutions to this print curl problem have been advanced. One proposed solution is to use conditioning rollers to add moisture to each sheet as it exits the system. This can be effected by passing the print sheet (substrate) through a nip formed by two pressure engaged rolls, a transfer roll and a backing roll. The transfer roll applies a wetting agent to the substrate as the substrate passes through the nip. These systems typically employ a metering roll to deliver evenly metered wetting agent (e.g., water) to the transfer roll which creates a wetted nip at the interface of the metering and transfer rolls.
One of the biggest problems encountered with conditioning devices is the sealing the wetting agent within the nip at the interface of the metering and transfer rolls to prevent water leakage from the rolls. When the wetting agent, which is supplied to the rolls continuously, leaks off the edge of the rolls, it creates a thick, non-uniformly metered section of agent on the transfer roll which then gets transferred to the substrate. This thick section of wetting agent then gets transferred to the substrate resulting in cockle or non-uniform curl. Thus, it is desired to provide a system to contain the wetting agent within the wetted nips and prevent leakage from the ends of the rolls.
The following disclosures may be relevant to various aspects of the present invention:
U.S. Pat. No. 5,987,301 to Acquaviva discloses a paper conditioner that includes a pump, supply lines, overflow lines, metering blade lines and return lines. A soaker hose is used to uniformly distribute conditioner agent to a wick. The soaker hose has pin-holes that are evenly spaced adjacent the wick. The wick is a high density material, such as, cotton that contacts and supplies the conditioner agent to metering rolls. These rolls contact donor rolls, which contact moisturizing rolls which contact the paper. Metering blades are used to remove excess conditioning agent from the moisturizing rolls.
U.S. Pat. No. 5,937,258 to Acquaviva et al uses one or more counter-rotating transfer rollers which are initially spaced from their respective back-up rollers in the intercopy gap, and which come together as the lead edge enters the nip area, and separates when the trail edge is about to pass. This permits the rollers to have a less demanding run out tolerance by bringing the rollers together when the sheet is in the nip, and then articulating the rollers apart in the intercopy gap.
U.S. Pat. No. 5,930,578 to Hwang discloses a paper conditioner to control image dependent curl in a copier/printer. The conditioner employs metering and transfer rolls which form a nip and have grooves positioned on the end portions of both rolls. Portions of both the metering and transfer rolls are located over a sump connected to a reservoir that supplies liquid to the nip. The grooves act as gutters and allow excess liquid to flow into the sump and subsequently back into the reservoir.
U.S. Pat. No. 5,434,029 to Moser describes an apparatus and method of preventing the curling of a substrate that has been subjected to heat for the purpose of fixing toner images to the substrate. Simultaneous constraint of the copy substrate and the application of moisture thereto is effected by passing the substrate through the nip formed by two pressure engaged rollers, one of which is utilized for applying the water to the back side of the substrate as the substrate passes through the aforementioned nip.
In accordance with one aspect of the present invention, there is provided a device for controlling leakage of liquid from a conditioning assembly. The device includes a transfer roll, for applying liquid to a side of a sheet, the transfer roll having an outer cylindrical surface; a metering roll, having an outer cylindrical surface in contact with the transfer roll, to control the flow of liquid to the transfer roll, the transfer and metering rolls being aligned with respect to one another along their axes so as to define a nip between the outer cylindrical surfaces; a supply pump to supply liquid to the nip; and a nozzle, positioned near an end of one of the transfer roll and the metering roll, generating an air pressure at the roll end.
Pursuant to another aspect of the present invention, there is provided a printing machine of the type wherein a sheet having indicia printed thereon advances through a conditioning system. The conditioning system includes: a pair of generally cylindrical rolls, the rolls being aligned with respect to one another along their axes so as to define a conditioner nip between the outer cylindrical surfaces, the pair of generally cylindrical rolls including a first roll adapted to drive a sheet in a first direction through the conditioner nip and a second roll, adapted to apply a liquid to a side of the sheet opposite the side that contacts the first roll, wherein the second roll rotates in a direction opposite the direction of the first roll; a metering roll in contact with one roll of pair of generally cylindrical rolls for forming a fluid nip therewith and controlling the amount of liquid supplied to the one of the rolls; a supply pump to supply liquid to the fluid nip; and an aperture, positioned near an end of the fluid nip, to generate a pressure differential between the end of the nip and an interior of the nip.
Pursuant to yet another aspect of the present invention, there is provided a method of controlling the amount of liquid within a nip formed between a metering roll and a transfer roll. The method includes supplying liquid to the nip and generating an air pressure at an end of the nip, the air pressure at the end of the nip being different than an air pressure at an interior of the nip.
Pursuant to yet another aspect of the present invention, there is provided a fluid sealing device for a system for supplying a measured amount of liquid to a transfer roll wherein the liquid is supplied to a fluid reservoir defined by the area of contact between the transfer roll and a metering device through a fluid supply hose, the metering device controlling the amount of fluid supplied to the transfer roll. The fluid sealing device comprising a nozzle positioned at an end of the fluid reservoir wherein the nozzle generates a pressure at the end of the fluid reservoir.